I don't have a problem with allowing races to colonize all types -- gas, rock, and ice. The way I deal with believability is to assume that the game's definition of a "Gas" planet is not what one normally thinks of as a gas giant. And same for Ice planet. Instead, the planets shown are all reasonably Earth-like, but represent extremes that one could not colonize with standard technology. So a Gas planet might be like Venus -- very thick atmosphere. An Ice planet might be like Mars.
This mental adjustment brings its own problems. For instance, how can there possibly be so many Earth-like planets in one star system??
